Conventionally, rigid polyurethane foams are used as structural members of vehicles or the like. Such rigid polyurethane foam is usually produced by blending a polyol mixture obtained by mixing an aromatic polyester polyol and a polyether polyol, and a polyisocyanate, and foaming the blended mixture.
In this application, the rigid polyurethane foam requires excellent mechanical strength and high open cell content. Therefore, the polyol mixture guarantees mechanical strength mainly with the aromatic polyester polyol and adjusts the closed cell content mainly with the polyether polyol, thereby ensuring these required properties.
For example, in order to produce a rigid polyurethane foam having an open cell structure with excellent mechanical properties, there has been proposed that a polyol mixture obtained by mixing phthalate polyester polyol having a hydroxyl value of 315 mg KOH/g, polyoxypropylene triol having a hydroxyl value of 281 mg KOH/g, and polyoxyethylene oxypropylene triol having a hydroxyl value of 28 mg KOH/g is used (see, for example, the following Patent Document 1).    Patent Document 1: Japanese Unexamined Patent Publication No. 2004-352835